Conventional Alumina Research Articles

Tin Dioxide as an Alternative to Platinum Promoter in Palladium-Catalyzed Wet Lean Methane Combustion

Tin dioxide was assessed as a substitute for part of platinum-group metals in a catalytic converter, and the activity of various Pd and PdPt catalysts was compared on conventional alumina support and SnO2 in wet lean methane combustion. Remarkably, Pd-only catalysts supported on SnO2 revealed higher activity compared with PdPt/Al2O3. The catalysts benefit from the strong metal-support interactions and high oxygen mobility in SnO2 with dual Sn4+/Sn2+ valency. SnO2 can thus be considered a potential replacement of Pt in a catalytic converter for a natural gas vehicle under lean burn conditions. This potentially decreases the price of the converter and eliminates the need for scarce and expensive Pt.

Influence of mesostructured alumina on the morphology of the active phase in NiWS/Al2O3 catalysts and their activity in hydrotreating of SRGO and VGO

Alumina supports were synthesized from aluminum hydroxide produced by the hydrolysis of secondary aluminum butoxide in the presence of the triblock copolymer Pluronic P123 (m-AlOOH) and from the commercial AlOOH pseudoboehmite powder (TH-AlOOH), as well as by mixing of m-AlOOH and TH-AlOOH gel in a ratio of 5:95 (m-TH-AlOOH sample). NiW/Al2O3 catalysts were prepared by impregnation of the synthesized supports with H3PW12O40 heteropolyacid and nickel citrate. Physicochemical properties of the supports and catalysts were investigated by low-temperature nitrogen adsorption, X-ray diffraction, NH3 thermo-programmed desorption, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. Catalytic properties were tested in hydrotreatment of straight-run gas oil and vacuum gasoil on a flow unit. The resulting hybrid support had a non-additively higher average pore radius (75 Å) and surface area (307 m2/g) than a conventional alumina support and hence preferred morphology of NiWS active phase (3.8 nm against 6.6 nm with the same stacking number) and an increase in the W sulfidation degree (68% against 54%) and Ni content in NiWS phase (44% against 26%). Therefore, the catalyst on the hybrid support exhibits higher activity in hydrodesulfurization, hydrodenitrogenation and hydrogenation of petroleum feeds, and a lower degree of deactivation and coke formation.

Interaction between binder and high silica HZSM-5 zeolite for methanol to olefins reactions

Aluminum phosphate (AlPO4) bound HZSM-5 extrudates were prepared with different Al/P ratios, and characterized with respect to the species chemical state, surface morphology, pore structure, and acidity, in comparison to conventional alumina bound and as-received HZSM-5 zeolite catalyst. The binder results in decrease in micropore volume implying blockage of micro channels within the zeolite crystal. AlPO4 binder forms dense amorphous matrix, while alumina forms a porous matrix with significant increase in mesopore volume and surface area. Moreover, both of the bound samples exhibit decreases in acidity strength and sites number, while the alumina bound one shows an increase in strong acidity strength which implies an enhancement of the acid stability of the strong acid sites. The reaction performance experiments on the simulated MTP operating conditions show that compared to alumina bound catalyst the AlPO4-bound type exhibits much lower paraffins and aromatics yields, and particularly that the polymer grade propylene may be easily obtained without C3 splitter, though the lifetime assessment reveals that alumina is much superior to AlPO4 as the binder.

Suspension high velocity oxy-fuel (SHVOF) spray of delta-theta alumina suspension: Phase transformation and tribology

Suspension high-velocity oxy-fuel (SHVOF) thermal spray is an emerging spray technology that enables the processing of nanometric feedstock. Although SHVOF thermal sprayed alumina coatings prepared from alpha alumina feedstock have been widely reported, a metastable δ-θ Al2O3 feedstock has yet to be investigated despite its low cost and commercial availability. In this study, an aqueous δ-θ Al2O3 suspension was sprayed on to a stainless steel (SS 304) substrate via SHVOF thermal spraying using an internal injection UTP TopGun. X-ray diffraction (XRD) of the as-sprayed coating showed δ-θ Al2O3 to γ-Al2O3 transformation upon spraying, along with amorphous/nanocrystalline phase formation. Furthermore, post-spray heat treatment of the coatings was performed at 600–750 °C for 6 and 48 h. The microhardness and indentation fracture toughness of the heat treated coatings increased by a factor of two compared to the as-sprayed coatings: due to grain refinement, pore consolidation and phase transformation of amorphous and γ-Al2O3 to δ-Al2O3. Unlubricated sliding wear tests were conducted at room temperature (~25 °C, relative humidity ~60%) using α-Al2O3 balls (Ø 9.5 mm) as the counter body at a normal load of 16.8 N to study the wear performance of the coatings. The wear rate of the as-sprayed coating and the coating heat treated at 600 °C for 6 h is of the order of 10−9 mm3 (Nm)−1, a noticeable improvement over conventional alumina coatings. The coatings heat treated at 750 °C for both 6 and 48 h failed abruptly by severe wear due to their pre-cracked surface.

High-Temperature Electrical Insulation Behavior of Alumina Films Prepared at Room Temperature by Aerosol Deposition and Influence of Annealing Process and Powder Impurities

Alumina (Al2O3) is a widely used material for highly insulating films due to its very low electrical conductivity, even at high temperatures. Typically, alumina films have to be sintered far above 1200 °C, which precludes the coating of lower melting substrates. The aerosol deposition method (ADM), however, is a promising method to manufacture ceramic films at room temperature directly from the ceramic raw powder. In this work, alumina films were deposited by ADM on a three-electrode setup with guard ring and the electrical conductivity was measured between 400 and 900 °C by direct current measurements according to ASTM D257 or IEC 60093. The effects of film annealing and of zirconia impurities in the powder on the electrical conductivity were investigated. The conductivity values of the ADM films correlate well with literature data and can even be improved by annealing at 900 °C from 4.5 × 10−12 S/cm before annealing up to 5.6 × 10−13 S/cm after annealing (measured at 400 °C). The influence of zirconia impurities is very low as the conductivity is only slightly elevated. The ADM-processed films show a very good insulation behavior represented by an even lower electrical conductivity than conventional alumina substrates as they are commercially available for thick-film technology.

Integrated piezoelectric vibration system for fouling mitigation in ceramic filtration membranes

Membranes used in separation processes are subjected to fouling which decreases the efficiency of the process and the membrane's lifespan. To reduce this fouling, vibrating membranes made from piezoelectrics, have recently been proposed. In this study we propose an innovative and flexible design where a conventional alumina membrane vibrates under the influence of electrically driven lead zirconate titanate (PZT) piezoelectrics, which are integrated outside the filtering area. This design makes it possible to isolate the piezoelectric and electrical interconnections from the fluid during the operation. The vibration system was analysed using “hard” and “soft” PZT compositions. Both the numerical model and the measurements of the membrane vibrations in air confirmed higher vibration amplitudes in the case of the “soft” PZT. We demonstrate that the vibration system with “soft” PZT actuators also resonates under water pressure, but at amplitudes lower than those measured in air. Filtration tests with humid acid showed that when the membrane is subjected to piezoelectric vibration, the decline in the flux was reduced by 59%, compared to the situation without the piezoelectric effect, suggesting that the novel vibration system with a built-in piezoelectric can be used in fluid-separation systems to minimise the fouling of ceramic membranes.

Copper-chromite glass fiber catalyst and its performance in the test reaction of deep oxidation of toluene in air

The paper is dedicated to the novel glass-fiber catalysts (GFC) using copper chromite as an active component for the reaction of deep oxidation of hydrocarbons and volatile organic compounds (VOCs). The catalyst support is the glass microfibrous fabric preliminarily modified by the addition of an external layer of silica secondary porous support. Surface thermosynthesis was applied for manufacturing of such catalysts. XRD and UV–Vis DRS studies have demonstrated that the active component in the synthesized GFCs is CuCr2O4 in the structural type of partially inverted spinel. As shown in experiments with the deep oxidation of toluene in air, the specific activity of CuCr2O4/GFC per unit mass of the active component exceeds that of the similar conventional CuCr2O4/Al2O3 catalyst by up to 20–30 times. Such significant rise is explained by both the much more efficient mass transfer in GFC cartridges and the higher intrinsic activity of the copper chromite in the GFC, where the particles of CuCr2O4 have the typical size of 10–25 nm compared to >100 nm in case of conventional alumina catalyst. The proposed GFC looks promising for the abatement of hydrocarbons and VOCs in different waste gases.

Lightweight alumina based fibrous ceramics with different high temperature binder

In this study, two kinds of alumina based fibrous ceramics, conventional alumina fibrous ceramics and the designed alumina/silica composite fibrous ceramic, were fabricated with different high temperature binders and the effect of the kind of high temperature binder and the binder content on the sample properties were analyzed. Furthermore, comparative analyses of the microstructure and compression and rebound properties of two kinds of alumina based fibrous ceramics were conducted. Conventional alumina fibrous ceramics were fabricated with the silica sol as the high temperature binder and the introduction of silica sol endow the sample a high compressive strength; however, the introduced silica phase would fill the pores constructed by the alumina fibers, consequently increasing the sample porosity. The alumina/silica composite fibrous ceramics were fabricated with glass fibers as the high temperature binder. The alumina fibers lapped with each other and acted as the main matrix of structure, while the molten silica binders bonded the junction of matrix fibers. Therefore, the high temperature binder would not influence the sample porous structure. The alumina/silica composite fibrous ceramic exhibited a higher compression and rebound ratio than the alumina fibrous ceramic due to its excellent skeletal structure.

Hydrodeoxygenation of pyrolysis oil for hydrocarbon production using nanospring based catalysts

Nickel (Ni) and ruthenium (Ru) decorated nanosprings (Ni-NS and Ru-NS) were prepared for use as potential hydrodeoxygenation (HDO) catalysts. The nanocatalysts were characterized by BET surface area measurements, electron microscopy and X-ray diffraction (XRD) and showed the NSs had a helical and mesoporous structure. The Ni and Ru decorated NSs showed good metal dispersity at the NS surface. Catalytic HDO conversion of phenol (model bio-oil compound) using NS were compared to conventional alumina (Al2O3) and silica (SiO2) gel catalyst supports. Ni–Al2O3 was easily deactivated in the presence of water while the Ni-NS catalysts performed very well irrespective of water being present. An increase in Ni loading (up to 50%) increased the Ni-NS activity while the high loading resulted in a detrimental effect on the activity of silica gel based catalysts. Ru based catalysts showed better activity and conversion on phenol HDO than Ni based catalysts, even in the presence of water. Ponderosa pine pyrolysis bio-oil was fractionated into water-soluble (WS) and water-insoluble (WI) fractions. The bio-oil WI fraction was first hydrocracked to lower its molar mass and then HDO treated with Ni-NS to successfully form cycloalkanes products. These NS based HDO catalysts show promise for upgrading pyrolysis bio-oils to biofuels.

Effects of non-spherical colloidal silica slurry on Al-NiP hard disk substrate CMP application

Spherical and non-spherical colloidal silica size and shape were characterized and its effects on aluminum alloy nickel plated (Al-NiP) hard disk substrate during chemical mechanical polishing (CMP) was investigated. Non-spherical colloidal silica slurry shows significantly higher material removal rate (MRR) with higher coefficient of friction (CoF) when compared to spherical colloidal silica of similar size. CMP evaluations on non-spherical colloidal silica slurry particle size distribution (PSD) reveal that MRR can be further increased by using wider PSD. Conventional slurry for Al-NiP hard disk substrates which use alumina–silica composite slurry induces embedded alumina thermal asperities (TA) defects which can cause reliability failure at product level. CMP comparison between conventional alumina–silica slurry and non-spherical colloidal silica slurry shows substrates polished by using non-spherical colloidal silica slurry have no embedded TA defects, lower surface roughness and lower surface defects.

Preparation of Nickel Phosphide Hydrodesulfurization Catalysts Assisted by Supercritical Carbon Dioxide

AbstractTo meet the increasing demand for clean fuels, deep hydrotreatment reactions are required, and thereby, improvement of the performance of the catalysts is highly desired. Silica‐supported nickel phosphide (Ni2P) nanoparticles have been reported to be efficient catalysts for hydrodesulfurization (HDS) reactions. We propose herein an innovative preparation method assisted by supercritical carbon dioxide (scCO2) to produce highly active Ni2P nanoparticles combining small size and high reducibility. In addition, scCO2 promotes the formation of the Ni2P phase with respect to the less active Ni12P5 phase. Such phosphide catalysts exhibit high activity for the HDS of 4,6‐dimethyldibenzothiophene, with a rate constant up to one order of magnitude higher than that of non‐supercritical references and close to conventional alumina supported NiMoS catalyst.

Unifying the templating effects of porous anodic alumina on metallic nanoparticles for carbon nanotube synthesis

Carbon nanotubes (CNTs) are a promising material for many applications, due to their extraordinary properties. Some of these properties vary in relation to the diameter of the nanotubes; thus, precise control of CNT diameter can be critical. Porous anodic alumina (PAA) membranes have been successfully used to template electrodeposited catalyst. However, the catalysts used in CNT synthesis are frequently deposited with more precise techniques, such as electron beam deposition. We test the efficacy of PAA as a template for electron beam-deposited catalyst by studying the diameter distribution of CNTs grown catalyst of various thicknesses supported by PAA. These are then compared by ANOVA to the diameter distributions of CNTs grown on metal catalyst supported by a conventional alumina film. These results also allow a unified description of two templating effects, the more common particles-in-pores model, and the recently described particles-between-pores.

Processing of near-net-shape dental crowns by conventional alumina slip casting in gypsum mould

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Comparing the Air Abrasion Cutting Efficacy of Dentine Using a Fluoride-Containing Bioactive Glass versus an Alumina Abrasive: An In Vitro Study.

Air abrasion as a caries removal technique is less aggressive than conventional techniques and is compatible for use with adhesive restorative materials. Alumina, while being currently the most common abrasive used for cutting, has controversial health and safety issues and no remineralisation properties. The alternative, a bioactive glass, 45S5, has the advantage of promoting hard tissue remineralisation. However, 45S5 is slow as a cutting abrasive and lacks fluoride in its formulation. The aim of this study was to compare the cutting efficacy of dentine using a customised fluoride-containing bioactive glass Na0SR (38–80 μm) versus the conventional alumina abrasive (29 μm) in an air abrasion set-up. Fluoride was incorporated into Na0SR to enhance its remineralisation properties while strontium was included to increase its radiopacity. Powder outflow rate was recorded prior to the cutting tests. Principal air abrasion cutting tests were carried out on pristine ivory dentine. The abrasion depths were quantified and compared using X-ray microtomography. Na0SR was found to create deeper cavities than alumina (p < 0.05) despite its lower powder outflow rate and predictably reduced hardness. The sharper edges of the Na0SR glass particles might improve the cutting efficiency. In conclusion, Na0SR was more efficacious than alumina for air abrasion cutting of dentine.

Microstructure and tribological properties of nanostructured and conventional plasma sprayed alumina–titania coatings

The nanostructured and two conventional alumina–titania coatings were deposited by atmospheric plasma spray. The presented studies show that nanoparticles are the predominant component of nanostructured Al2O3–13TiO2 powder grains. The microstructure of coating consisted of two distinct regions: fully melted and unmelted or partially melted nanostructured areas, which were comprised of components of a starting powder. Microhardness and modulus of these coatings were significantly lower than the values obtained for nanostructured coating. It was found that the plasma-sprayed nanostructured Al2O3–13TiO2 coating possessed better tribological properties than those of conventional alumina–titania coatings. The lower coefficient of friction and wear of the nanostructured Al2O3–13TiO2 coating is attributed to the bimodal microstructure and the enhanced mechanical properties in comparison to conventional alumina–titania coatings.

Sliding wear behavior of in-situ FeAl2O4 matrix nanocomposite coating fabricated by plasma spraying

FeAl2O4 matrix nanocomposite coating was successfully developed by plasma spraying Al/Fe2O3 composite powders. The microstructure of the coating was characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, respectively. The results indicated that the as-prepared coating exhibited dense microstructure with nano-sized grains FeAl2O4 matrix and α-Fe and Al2O3 phases. The microhardness, toughness, and especially unlubricated sliding wear behavior of the composite coating were investigated. The toughness and sliding wear resistance of the composite coating were significantly enhanced despite its lower microhardness compared with the conventional alumina coating, which was attributed to the dense microstructure, the inclusion of ductile metallic phase in the composite coating, and the nanostructured microstructure of the composite coating. The worn surface of the alumina coating was characterized by intergranular fracture and interlayer fracture. The fracture of the composite coating was lower than that of the alumina coating. Intergranular fracture and interlayer fracture were dominated in the low load wear process of the composite coating. Adhesive wear was dominated in the high load wear process of the composite coating and accompanied by abrasive wear.

Effect of Reactive Alumina on the Physico-Mechanical Properties of Refractory Castable

The effect of reactive alumina and its different proportions on physico-mechanical properties of conventional high alumina castables were studied. The castables with different percentages of reactive alumina were prepared by using calcium aluminate cement as a binder and various refractory properties of the prepared castables, i.e. apparent porosity, bulk density, cold crushing strength, hot modulus of rupture were studied. The mineralogical analysis was done by X-ray diffractometry, whereas morphological analysis was carried out by scanning electron microscopy. The results showed that the castable with high percentage of reactive alumina hydrated early as well as developed early sintered strength due to its faster rate of hydration and higher reactivity at lower temperature. The hot modulus of rupture was also found to be higher in the castable with more reactive alumina. Mineralogical analysis showed well developed mullite and corundum phases in the refractory concrete which was supported by morphological analysis.

Thin Porous Alumina Sheets as Supports for Stabilizing Gold Nanoparticles

Thin porous alumina sheets have been synthesized using a lysine-assisted hydrothermal approach resulting in an extraordinary catalyst support that can stabilize Au nanoparticles at annealing temperatures up to 900 °C. Remarkably, the unique architecture of such an alumina with thin sheets (average thickness ~15 nm and length 680 nm) and rough surface is beneficial to prevent gold nanoparticles from sintering. HRTEM observations clearly showed that the epitaxial growth between Au nanoparticles and alumina support was due to strong interfacial interactions, further explaining the high sinter-stability of the obtained Au/Al2O3 catalyst. Consequently, despite calcination at 700 °C, the catalyst maintains its gold nanoparticles of size predominantly 2 ± 0.8 nm. Surprisingly, catalyst annealed at 900 °C retained the highly dispersed small gold nanoparticles. It was also observed that a few gold particles (6-25 nm) were encapsulated by an alumina layer (thickness less than 1 nm) to minimize the surface energy, revealing a surface restructuring of the gold/support interface. As a typical and size-dependent reaction, CO oxidation is used to evaluate the performance of Au/Al2O3 catalysts. The results obtained demonstrated Au/Al2O3 catalyst calcined at 700 °C exhibited excellent activity with a complete CO conversion at ∼30 °C (T100% = 30 °C), and even after calcination at 900 °C, the catalyst still achieved its T50% at 158 °C. In sharp contrast, Au catalyst prepared using conventional alumina support shows almost no activity under the same preparation and catalytic test conditions.

Metal dispersion, accessibility and catalytic activity in methane oxidation of mesoporous templated aluminosilica supported palladium

Palladium catalysts using templated mesostructured porous silicas and aluminosilicas of MCM-41 type as supports were synthesized with various metal loadings and particle sizes as well as different metal accessibilities to the reactants. The metal was deposited by reacting an aqueous solution of [Pd(NH3)4]2+ complexes with the support, the template of which was partially extracted. The evolution of the support characteristics was monitored at different steps of preparation using X-ray diffraction (XRD), N2 physisorption and transmission electron microscopy (TEM). Particle size and metal accessibility were evaluated combining information from XRD line broadening, TEM observations, UV–visible spectra and H2 uptakes. The PdO reducibility was investigated using temperature-programmed reduction (TPR). In the templated mesoporous aluminosilicate, 27Al-MAS-NMR revealed that Al3+ occupied mostly tetrahedral sites, a fraction of which adopted an octahedral environment in the presence of palladium. According to the light-off curves, the catalytic methane oxidation activity is enhanced in pure silica supports where partial pore wall collapse has occurred. In contrast, for aluminosilica supports the beneficial effect of Al3+ on metal dispersion and catalytic activity was counterbalanced by partial metal encapsulation. Optimizing palladium particle size and avoiding as much as possible metal encapsulation give rise to catalysts more active than the conventional alumina supported palladium catalysts.

Mechanical Properties of Transparent Polycrystalline Alumina Ceramics Processed Using an Environmentally Benign Thermal Gel Casting Process

Technological advancements in ceramic powder synthesis, shaping and sintering have made it possible to tailor the microstructural, mechanical and optical property relationships in the case of advanced transparent ceramic materials. Transparent polycrystalline alumina (TPCA) is the hardest known transparent ceramic and one of the emerging candidate materials for transparent armour applications. The prerequisites for obtaining transparency with the high hardness, is to achieve the sintered average grain sizes 96 % density at an optimum temperature of 1350 °C. The post sintering by Hot Isostatic Pressing (HIP) at an optimum temperature of 1350 °C and a pressure of 195 MPa resulted in >99.5 % of the theoretical density and a grain size of 0.7 μm. For the sake of comparison, conventional polycrystalline alumina samples (non-transparent) were also processed by sintering at 1550 °C under PLS condition with nearly the same densities (designated as PCA). The TPCA thus developed exhibit a combination of high hardness of 21 GPa, flexural strength of 550 MPa and excellent fracture resistance properties as compared to conventional PCA samples.